This program is a continuation of NIH R29 HL.55686, extending optical coherence tomography (OCT) imaging to the evaluation of pulmonary arterial and venous circulation. OCT is a recently developed method of high resolution imaging which was developed for the evaluation of unstable plaque through this program. OCT is analogous to ultrasound, measuring the intensity of backreflected infrared light rather than sound. It's resolution, between 4 and 20 um, is up to 25 X higher than anything available in clinical medicine. Among the observations under NIH R29 HL1155686, OCT demonstrated a feasibility for the identification of vulnerable plaque, superior resolution to high frequency ultrasound (IVUS), and in vivo imaging in a rabbit model. In addition, a catheter based delivery system and high speed data acquisition system were developed. The technology is now being commercialized for in vivo human imaging. In this proposal, we will be extending OCT imaging to the pulmonary vasculature. In particular, in addition to +/-aracterizing normal pulmonary vasculature, a focus will be placed on the evaluation of pulmonary hypertension and the guidance of pulmonary vein ablation for atrial fibrillation. Pulmonary hypertension, an increase in pulmonary blood pressure, is a leading cause of mortality worldwide. Current methods of assessing the pulmonary vasculature are inadequate or dangerous. A method of high resolution assessment of the vasculature could be a powerful tool in the management of this disorder. Atrial fibrillation is a common disorder associated with a high incidence of stroke. It has recently been demonstrated that a significant portion of atrial fibrillation can be cured by ablation of foci in the pulmonary veins. However, this procedure is associated with a high incidence of pulmonary vein stenosis. A technology capable of controlling the degree of ablation could substantially improve the outcome of this procedure. The hypothesis of this proposal is that OCT demonstrates a feasibility for high resolution assessment of pulmonary vasculature, both for diagnosis and guiding interventions. The hypothesis will be tested through the following specific aims: Aim 1. Development of an OCT Imaging Catheter, Aim 2. Assessing the Pulmonary Veins Ablation in an Animal Model, Aim 3. Vascular Assessments In Vitro, Aim 4. OCT Elastography and Aim 5. OCT Imaging will be performed in an Animal Model of PTN.